The prehistoric farmers of southern Mexico must have longed for a miracle. A tropical climate made their fertile valleys nearly ideal for planting, despite elevations approaching 2,000 meters, and heavy rains ensured bountiful crops during the six-month monsoon season. Under such favorable conditions, this region became the cradle of New World agriculture and the birthplace of corn. Yet these early agriculturalists faced one crucial limitation: during half the year, the weather was too dry for farming. With a year-round water supply, their hand-tilled fields might yield two or even three harvests annually. But how could the farmers get more water?

Their solution was not a miracle but a marvel of human ingenuity: large-scale engineering projects designed to store and transport water. From modest origins that left few traces, construction gradually progressed to a monumental scale. The Purr¿n Dam, for example, which was built in the Tehuac¿n Valley starting around 750 B.C., measured 400 meters long, 100 meters wide and nearly 25 meters high. Workers transported by hand, a few kilograms at a time, some 2.64 million cubic meters of earth. This dam probably remained the largest water retention structure in the Americas until the 18th century. Nearby, ancient engineers built thousands of kilometers of canals and aqueducts that predated the arrival of Europeans in Mexico by two millennia. They diverted water from springs and streams, conveying it across drainage divides, around canyons and down steep slopes. Other resourceful inventions collected rainwater from buildings and plazas. The people of southern Mexico exploited virtually every source of water in their environment.